1. Field of the Invention
The present invention relates to a glass antenna device for an automobile provided on a rear window glass sheet of automobile, in particular, it relates to a glass antenna device for an automobile which is suitable for receiving AM broadcasting wave signals and for improving remarkably signal receiving sensitivity.
2. Discussion of Background
Generally, a glass antenna device for an automobile as shown in, for example, FIG. 17 has been known as an antenna device for receiving radio waves, which is mounted on an automobile.
In a rear window glass sheet 1 for an automobile, generally, a defogger 4 comprising heater strips 2 and bus bars 3a, 3b, 3c for feeding a current to the heater strips 2 is provided, and antenna conductors 5 are provided in an upper region of the rear window glass sheet 1 other than the region where the defogger 4 is provided.
The antenna conductors 5 have a pattern to function as an antenna which is formed by a method wherein a conductive metal-containing paste such as an electric conductive silver paste is printed on a surface of the rear window glass sheet 1 facing the interior side of the automobile, followed by baking. A signal received by the antenna conductors 5 is connected through a power feeding terminal 6 to a preamplifier 20 where the signal is amplified, and then, the amplified signal is transmitted to a receiver 40 through a feeder 30.
In the glass antenna device of this kind, a d.c. power 12 is connected to a positive side bus bar 3a of the defogger 4 by interposing a switch 11. In order to form an antenna of high gain in receiving AM broadcasting signals, a choke coil 7 having an inductance value of, for instance, about 1 mH is interposed as a noise filter between the d.c. power source 12 and the bus bars 3a, 3b of the defogger 4 so that the defogger 4 is maintained in a high frequency state with respect to the automobile body as the earth to thereby maintain a received signal voltage induced in the defogger 4, and at the same time, the distance L between the antenna conductors 5 and the defogger 4 is rendered to be about 5-10 mm so that a voltage induced in the defogger 4 is brought to a capacitive coupling relation to the antenna conductors 5. Further, in order to absorb noises generated in the automobile body which may enter into the d.c. power source 12, a capacitor 9 is connected as a noise filter in parallel to the d.c. power source 12.
In the conventional glass antenna device, however, it is essential to use the choke coil 7 having a large inductance value of about 1 mH in order to form an antenna of high gain to receive AM broadcasting wave signals. Use of such choke coil deteriorates productivity.
Further, since it was necessary to maintain the defogger 4 in a high frequency state in receiving the AM broadcasting wave signals, the defogger 4 had a high impedance with respect to the automobile body whereby the defogger 4 was apt to suffer noises from the automobile. In order to prevent such disadvantage, it was necessary to add a noise-preventing device for electrical equipments for the automobile. The noise-preventing device decreased the performance of the glass antenna device and productivity was inferior.
In addition, in the conventional glass antenna device, the choke coil 7 had to be connected to each of the positive side bus bar 3a and the negative side bus bar 3b. Accordingly, it was recommended to arrange the positive side bus bar 3a and the negative side bus bar 3b in a vertically separated state at an end side of the heater strips 2 from the standpoint of facilitating the wiring of the power source line 15. Accordingly, it was difficult to apply the above-mentioned arrangement to a defogger which is so designed that the positive side bus bar 3a and the negative side bus bar 3b are separately arranged at both sides of the heater strips 2, and therefore, flexibility in designing the defogger 4 was limited.